Plasma membranes of macrophages and lymphoid cells, both normal and of tumor origin, bear receptors for IgG (Fc receptors). These receptors are vital to phagocytosis of antibody-coated particles and to antibody-mediated cell-cytotoxicity (ADCC). They are implicated as well in a number of biological processes including regulation of antibody production, modification of cytotoxic and mitogenic responses, stimulation of suppressor cell function, and mediation of enhancing effects of immune-complexes on tumors and allografts. A more precise knowledge of the structure of these receptors is needed. I am proposing to isolate and characterize Fc receptors on mouse and human macrophages and lymphocytes, both normal and from tumors. Established methods which have been successful in preliminary experiments will be utilized. Techniques for radiolabeling membrane macro-molecules, purifying membranes, lysing membranes with detergent, and purifying receptors by affinity chromatography will be used. I will then characterize the receptors as to molecular weight and composition for purposes of analyzing the suspected heterogeneity of Fc receptors. Antisera against the purified receptors will be prepared both for the purpose of purifying large amounts of solubilized Fc receptor for sequence analysis and to delineate Fc receptor function. For purpose of studying Fc receptor function, Fab or F(ab')2 antisera specific for Fc receptors will be utilized to block or modulate Fc receptor activity in several in vitro assays of immune function. The results of these studies will allow a more precise understanding of the contribution of Fc receptors to the various physiological and pathological mechanisms outlined above.